can you mix concrete and plywood substrates under tile?

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  #1  
Old 05-02-17, 06:13 PM
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can you mix concrete and plywood substrates under tile?

I'm repairing our kitchen floor and replacing the tile. Originally the tile was resting on 1/2" ply which was on top of 5/8 subfloor.
Most of the 1/2" ply had to be removed but there is a section left. I'd like to use concrete backer board on the new tile but I don't want to have to go through the effort of removing the remaining (exisiting) plywood if I don't have to.
Tried googling, but just couldn't find anything that would answer this.
So, does anyone know if it would be safe to leave the existing ply and use concrete backer butted against it?

So basically about 2/3 the floor would have 1/2" hardiboard and the rest is 1/2" ply, all covered with 12x24 tile. Fairly small kitchen (about 80sqft).
Or do I need to rip out the remaining ply and keep it uniform? The joists seem to be 12" oc based on the subfloor nail pattern (can't see it to confirm though).
Please advise if you know.
Thanks.
 
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Old 05-02-17, 06:23 PM
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You can't put tile directly on the plywood. You need some kind of decoupler under ceramic tile.
 
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Old 05-02-17, 07:07 PM
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Tile on plywood is acceptable - just not best practice. It just shouldn't be laid on the subfloor directly. But you can do subfloor, second layer of plywood, tile.
The tile I took off had been on the plywood for over 15 years. Not a tile out of place.
My concern is about having the disparate substrates. Not sure if that's doable in this scenario. Concerned it has to be either or.
 
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Old 05-03-17, 01:07 AM
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Sorry, but tile directly on plywood is not acceptable. You need a decoupler or any movement and the tile will fail.
 
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Old 05-03-17, 01:31 AM
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I think what Sam is saying is that "Can it be done?". The answer is probably yes. Is it the recommended or best way, including instructions by tile makers, probably not.

I'm not a tile guy, but I did it when trying to sell my old place. Yes, it went down ok, and was solid. Knowing what I know now, no way in he11 would I have done it. The new owners probably had to have it ripped up a year later. By the time I was moving, there were already cracks in the grout and a few "crunchy" spots.

It seems like a lot more work than just getting up that last section of ply overlay and doing the whole floor the best it can be.

If you got such a small area, what's the big deal with ripping up that last section and doing it all the same?

Sam has been doing it for over 40 yrs, I think I'd trust his advice.
 
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Old 05-03-17, 04:03 AM
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22 yrs ago I put in a tile hearth for my wood stove. A tile man I was doing some painting for got me the materials and gave me instructions for installation. He had me lay the tiles directly to the plywood although he did give me a jug of additive to mix into the thinset. The tile and grout still look good today .... but maybe I was just lucking. Like Vic, knowing what I now know - I wouldn't do it that way again!!!

You say the subfloor is only 5/8" It's quite likely that isn't thick enough for your tile. IMO it would be a better idea to add 1/2" ply as needed and then put down 1/4" cement board.
 
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Old 05-03-17, 07:13 AM
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The problem is the plywood absorbs moisture from the thinset making for a weak bond. Does it work sometimes? Sure. Would I do it? Not likely.
 
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Old 05-03-17, 07:17 AM
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yeah, I've decided to just rip it out. After posting this last night I thought about it and just decided it just bugged me too much to leave it even if it was doable.

the 5/8 subfloor plus 1/2 backer meets the recommended thickness as per requirements I was given. Thicker would be nice but would raise the floor too much compared to surrounding hardwood.
 
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Old 05-03-17, 08:29 AM
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Cement board adds no structural value!! That's why I suggested keeping/adding 1/2 over the 5/8. Then 1/4" cement board is sufficient or you could use Ditra which doesn't really add any height.
 
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Old 05-03-17, 10:33 AM
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What Mark said. Plus, have we evaluated the floor joists yet?
 
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Old 05-03-17, 11:14 AM
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I would go to the John Bridge tile forum. He has a chart to use to determine if the floor will work for tile.
 
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Old 05-03-17, 05:45 PM
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Captain,

Glad you've decided to rip it all out, after all, it's only a total of 80 ft.

5/8 subfloor is pretty wimpy, but if the joists are @12" oc, you're good. Your Hardie adds almost nothing to the values, but 5/8" ply is considered adequate for ceramic tiles. I see no reason to use Hardie 500 which is .42" thick, 1/4" is better for floors. I recommend Ditra, but it's a bit more $$$ and would end up thinner than the hardwood.

Jaz
 
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